There is a broad range of applications for power transmission or conveyor chain made of an economical, corrosion resistant material, having a very high strength-to-weight ratio. For example, in grain and cement elevators, the weight of the chain used to lift bulk materials is itself a major contributor to the load on itself and the drive system. In food processing or underwater operations, corrosion resistance is an essential requirement.
There has also been a long standing need for an improved sludge collector chain to operate under conditions of the hostile corrosive embodiment of sludge collectors and where heavy chain loads are experienced. It is also important in those applications that the chain have dimensional stability. Sludge collector chains are typically employed in pairs of parallel strands which between them support elongated flights. The flights travel under the pull of the chains to scrape the accumulated sludge settled along the bottom of a sewage treatment tank. Such tanks typically range in length up to 350 feet, and in depth up to 15 to 20 feet. The sprocket driven, submerged chains for these tanks are quite long, endure heavy tensile loads and operate in a corrosive environment.
Initially sludge collector chain was formed of malleable cast iron which was very heavy and unwieldly, required considerable power to drive it, and was vulnerable to corrosion by most wastewater. Such cast chain is still used in many larger settling tanks because of the heretofore unavailability of a non-metallic chain having adequate pulling strength.
More recently chain manufacturers have struggled to replace the heavy cast metal with corrosion resistant non-metallic materials, particularly plastics. Plastic chain offers the advantage of lower weight which thereby reduces chain load, power consumption, wear, and installation costs. Although a few of these plastic chains, representatives of which are discussed below, have proven acceptable in some smaller sludge collector applications, none of the heretofore commercially available ones have had enough strength to operate in the larger tanks, for example, tanks having a length greater than 225 feet.
U.S. Pat. Nos. 4,123,947 and 4,250,764 are illustrative of typical plastic sludge collector chains. These chains comprise a series of single piece, hermaphroditic links made of a synthetic resin and connected together by non-metallic pins. The chain design shown and discussed in U.S. Pat. No. 4,272,952 is similar except that the chain link is comprised of two identical half links permanently joined together. The complexly shaped links of these prior chains are made by injection molding.
The success of these previous plastic sludge collector chains has been limited because of their limited pulling strength. None of these commercial chains has an ultimate breaking strength greater than about 7,000 lbs. force, and these chains can only be used in lighter duty applications. None come close to having a strength approaching that of cast iron chain.
As mentioned variously in the aforementioned patents, persons skilled in the art heretofore recognized that higher strengths could be achieved with glass fiber reinforced resin or equivalent composites; but they were constrained in their effective use of such composites by concerns that: (a) such stronger composites cannot be molded into the complex shape of conventional sludge collector chain; (b) such composites were too highly abrasive and therefore produce excessive wear on mating parts; or (c) that any conceptually suitable composite chain design would be too costly.
The plastic chain links shown in the three aforementioned patents have in common with their predecessor cast metal chains, a complex shape including curved sidebars with widened bearing surfaces or edges intended to engage annular rims on so called "chainsaver" sprockets. The only practical method of forming synthetic resins into such intricate shapes is by molding, typically injection molding. The types of reinforcement that can be used effectively in injection molding is limited. In the case of fibers, they must be discontinuous and usually fairly short. Also, because the fibers flow with the resin in the mold, it is difficult to control their final orientation to effect maximum reinforcement.
However, the prevailing property needed in a material for the links themselves, and Particularly the sidebars, is tensile strength. In the case of fiber reinforced materials, the optimum tensile strength results when the high strength filaments are aligned parallel to the direction of pull, and particularly if the filaments are continuous.
The art of filament winding a ring or looped member to take advantage of the foregoing reinforcement principle was tried on other kinds of chain. The patentees of U.S. Pat. Nos. 3,153,898 and 3,733,811 employed a filament wound, fiber reinforced plastic to make links for an anchor chain "with a strength surpassing that of welded steel". However, this type of chain is not subject to repeated cyclical articulation between connected links, and therefore not vulnerable to wear due to the abrasiveness of the composites. Accordingly, this anchor chain technology is not readily transferable to articulating conveyor chain such as sludge collector chain.
German Patent No. 1,135,721 illustrates and discloses a sprocket driven transmission type chain having sidebars with filament wound, reinforced plastic cores. The sidebars are formed by winding filaments through a slit in a loop-shaped mold which then becomes a non-abrasive jacket for the finished link. This chain construction is not adapted to a design for sludge collector chain, as evidenced by the later issued U.S. Pat. Nos. 4,123,947; 4,250,764 and 4,272,952, which represent the practical state-of-the-art. Furthermore, as will be more apparent from the detailed description of the present invention, the method of making a chain as taught in German Patent 1,135,721 is too costly to be competitive with the chain and process taught herein.
Attention is also directed to German Patent 34 08 295 illustrating a non-metallic fiber wound chain including spacers separating the chain links from the chain pin.
Thus, despite the existence of some knowledge of using filament winding to reinforce plastic links for chain, the designers of conveyor chain, particularly sludge collector chain, struggled along until now with the material constraints imposed by their conventional wisdom dedicated to a classified configuration which required molding. They occupied themselves with improving pin characteristics or connectibility between links, and apparently conceded that a truly high strength chain of this type couldn't be made at a realistic cost.
It is therefore a principal object of the present invention to provide a chain links of consistent quality and strength for a truly high strength, non-metallic transmission or conveyor chain at a competitive cost.
It is a more specific objective of the present invention to provide a commercially affordable non-metallic chain link assembly suitable for sludge collector chain, which is stronger than any of the previously available non-metallic chains suitable for that application.
It is another object of the present invention to provide a link assembly having sidebars of a non-metallic material reinforced with high strength filamentary material oriented to maximize the tensile strength of the sidebar to yield a chain having a high strength-to-weight ratio.
It is another object of the invention to provide a plastic chain having strong and long lasting sidebars and high strength wear resistant connecting pins.
It is another object of the present invention to provide a high strength plastic chain link assembly comprising a minimum of components which is easily assembled, disassembled and/or connected to other links, without the need of special tools.
It is a still further object of the invention to provide a method of making a high-strength, non-abrasive, reinforced plastic connecting pin for a transmission or conveyor chain.